CD40-CD40 ligand (CD40L) interaction plays an important role in macrophage/monocyte-mediated inflammatory processes by up-regulating cytokine production by macrophages/monocytes and by preventing macrophage apoptosis at the inflammation sites. The present study investigated the possible regulation of CD40L expression in peritonitis during continuous ambulatory peritoneal dialysis (CAPD). We used fluorescence-activated cell sorter (FACS) analysis to detect CD40L expression on macrophages obtained from peritoneal dialysate. Our results showed that CD40L expression on macrophages was significantly increased in a peritonitis group (4.62 +/- 6.54) as compared to a control group (0.76 +/- 0.30, p < 0.01). The CD40L-positive cells were also significantly increased during peritonitis (97.86% +/- 1.67% in the peritonitis group as compared to 73.10% +/- 26.94% in the control group, p < 0.05). After successful treatment, the expression of CD40L was significantly reduced (3.66 +/- 1.12 vs 1.05 +/- 0.02, p < 0.05). We conclude that functionally expressed CD40L on macrophages may take part in acute inflammatory response during peritonitis in CAPD and may play an important role in the local defense against infection in the peritoneal cavity.